There has been known an exhaust gas purification system that is provided with a NOx storage reduction catalyst in order to purify nitrogen oxides (NOx) in an exhaust gas discharged from an internal combustion engine, in particular an internal combustion engine in which lean burn combustion is carried out. This NOx storage reduction catalyst is a catalyst which performs the purification of an exhaust gas by occluding NOx in the exhaust gas into the catalyst in cases where an atmosphere around the catalyst is in a high oxygen concentration state, and reducing the occluded NOx in cases where the atmosphere around the catalyst is in a low oxygen concentration state with the presence of a reducing agent.
Sulfur oxides (SOx) in the exhaust gas as well as NOx therein are also occluded in the NOx storage reduction catalyst, so SOx poisoning occurs in which the exhaust gas purification function of the NOx storage reduction catalyst decreases in accordance with the increasing amount of SOx occlusion.
Accordingly, in order to eliminate the SOx poisoning of the NOx storage reduction catalyst, a control operation to cause the SOx occluded in the catalyst to be released therefrom (hereinafter, referred to as “SOx poisoning recovery control”) may be carried out by raising the catalyst bed temperature of the NOx storage reduction catalyst to a temperature at which the release of the SOx can be made, and at the same time supplying a reducing agent so as to make the surrounding atmosphere of the NOx storage reduction catalyst into a stoichiometric or rich atmosphere.
In relation to this, there has been proposed a technique of changing the air fuel ratio of an exhaust gas (hereinafter, referred to as “the exhaust gas air fuel ratio”) flowing into the NOx storage reduction catalyst to a richer side in accordance with the decreasing amount of SOx occluded in the NOx storage reduction catalyst in the course of the SOx poisoning recovery control (see, for example, a first patent document).
[First Patent Document] Japanese patent application laid-open No. 2005-90253
[Second Patent Document] Japanese patent application laid-open No. 2003-293747
[Third Patent Document] Japanese patent application laid-open No. 2005-291039